There are applications wherein both the output frequency and sensitivity of a measuring device are proportional to the rate at which the device is spinning. One class of such devices is disclosed in U.S. patent application Ser. No. 528,243 filed on Nov. 29, 1974 for "Apparatus for Performing Inertial Measurements Using Translational Acceleration Transducers and for Calibrating Translational Acceleration Transducers" and assigned to the assignee of the present application. In this application there is disclosed apparatus for measuring angular velocity by spinning one or more translational acceleration transducers, at least one of which is placed parallel to the spin axis, spinning around an axis perpendicular to the turn axis. The transducer output signal = K x turn rate x spin rate x cos (2.pi.x spin rate x time) where K is a constant depending upon system geometry and transducer sensitivity. It is thus seen that the output signal is a cosine wave of the same frequency as the spin rate with an amplitude proportional to the turn rate multiplied by the spin rate. Thus, to achieve accurate turning angular rate measurements using such a device it has been necessary to know the spin rate by either maintaining the spin rate of the transducer constant or by independently measuring it and making suitable adjustment.
In many applications this may be no problem as the transducer can be spun using synchronous or other regulated motors. However, one of the objects of the device is to provide inexpensively a gyroscope substitute. To do same inexpensively necessitates for some applications the use of an inexpensive spin motor such as a spring wound or dc electric motor. When such an unregulated motor is used, of course, the transducer is not spun at a constant rate. Also gas driven or impulse started turn rate indicators for boost-glide missiles where the system is given an initial excitation and then allowed to coast, thereby having an unknown, changing spin rate, necessitates spin rate measurement. Measurement of the changing spin rate will increase the cost and in certain application may be impractical.
Also in the aforementioned patent application there is described an application using spinning translational acceleration transducers on a spinning projectile to make inertial measurements. In this application the transducers are spun by the spinning projectile itself and therefore, no additional spin motors are required. However, since a spinning projectile spins at a continuously decreasing rate during its trajectory, it is necessary to know the instantaneous spin frequency thereof to make accurate measurements. This requires the use of additional transducers on the spinning projectile to make such measurements or other spin measuring means. This can be both relatively expensive and difficult.